1. Field of the Invention
This invention relates to measurement of low levels of phosphonate materials, particularly in brine. The process of this invention stabilizes the phosphonate sample, oxidizes the hydrochloric acid acidified phosphonate to orthophosphate in the presence of an oxidizing agent with mild heat and pressure and uses organic solvent extraction, preferably with methyl iso-butyl ketone/cyclohexane or ethyl acetate, to separate the phosphorous containing compound. The process of this invention is suitable for measurement of phosphonate concentrations of about 0.1 to about 10 milligrams per liter in gas and oil well brines with reproducibility of plus or minus about 5 percent.
2. Description of Related Art
Phosphonates are well known for their use as scale and corrosion inhibitors in gas and oil wells. Such wells usually coproduce large volumes of brine. Low concentrations of phosphonates, in the order as low as 0.16 mg/l phosphonate, can effectively prevent scale formation in operating wells. Matty, J. M., K. Vaughese, G. G. Waggett, M. B. Tomson and L. A. Rogers, Control of Scale Associated with Geopressured-Geothermal Brine Production, 6th Gulf Coast Geopressured-Geothermal Energy Conference on Mitigation of Scale Formation in Geothermal/Geopressured Energy Production, pgs. 137-147, (1985); Rogers, L. A., K. Varughese, S. M. Prestwich, G. G. Waggett, M. H. Salimi, J. E. Oddo, E. H. J. Street and M. B. Tomson, Use of Inhibitors for Scale Control in Brine Producing Gas and Oil Wells, SPE Prod. Eng. J., pgs. 77-92, (February 1990) The decision to re-treat a well is frequently not based upon well performance criteria, but upon measurement of inhibitor concentrations. Therefore, the ability to measure concentrations of phosphonates down to a range of about 0.1 to about 1.0 mg/1 in brine becomes important to the decision to re-treat a well. The operational cost and loss in production during shut-ins necessary for such re-treatment is significant and is desired to be kept to minimum in good well management, even though the cost of treatment chemicals may be small.
The drop in phosphonate concentration following treatment of a well is not linear and is not entirely predictable with time following treatment. In fact, phosphonate concentration in the well fluids quickly drops from initial treatment concentration of about 10,000 mg/1 and greater to about 1 mg/1 or below and then levels off to a relatively steady state value of about 0.1 to about 1.0 mg/l. This relatively steady state value depends upon the brine chemistry, the type of formation, and the inhibitor chemistry. Although phosphonate levels of about 0.1 to 1.0 mg/1 may be too low for effective corrosion control, they are generally adequate for scale control in production tubing and thus, from an economic standpoint, are satisfactory for continued well operation in these concentration ranges. Again, this points out the desirability of accurate measurement of phosphonate concentrations at levels of about 0.1 to about 1.0 mg/l.
Prior analytical methods for measurement of the concentration of phosphonates in brine have included:
1) Use of radioactive labelled carbon or phosphorous. These methods, which require sophisticated instrumentation, are in the developmental stages and further present the problems of availability and handling of the radioisotope as well as requiring permitting.
2) Complexation with copper, thorium, iron or magnesium. Methods of complexation of phosphorous with these metals are susceptible to numerous interferences by trace metals.
3) Inductively coupled plasma arc mass spectrometry.
4) High performance ion chromotography with or without ion suppression. Both methods 3) and 4) are in the developmental stage and require sophisticated instrumentation not normally available in the field. Also, the high salinity of the natural brine frequently interferes with these methods for phosphate analysis.
5) Oxidation of phosphonate to phosphate and colorimetric measurement of increased phosphate concentration. These methods are easy to perform, but are susceptible to interferences from trace amounts of materials, such as, calcium, barium, strontium, sulfate, sulfide and silicate as well as major ion interferences due to ions such as chloride. These methods are also susceptible to interference caused by turbidity. Such interferences render this method for measuring phosphonates in brine solutions limited to those containing over about 5 mg/l phosphonate, resulting in re-treatment of the well considerably before scale would begin to develop.
The analytical method most commonly used today for measurement of phosphonate concentration is the "Hach Method" developed at Hach Chemical Company. Kindel, L. E., Determination of Low Concentrations of the Dequest Products Via Persulfate Digestion, Special Report No. 7823, Monsanto Industrial Chemicals Company, St. Louis, Mo. 63116, (January 7,1972) In the Hach Method, phosphonates are oxidized to phosphate by UV/persulfate, and the increased phosphate concentration measured as the phosphomolybdate blue complex. Standard Methods for measurement of phosphates in natural and waste waters using the phosphomolybdate blue complex is described in Cleseri, L. S., A. E. Greenberg, R. R. Trussell, Standard Methods for the Examination of Water and Wastewater, 4500-P Phosphorous, pgs 4-166-4-181, Published by American Public Health Association, American Water Works Association, and Water Pollution Control Federation, Washington, D.C., (1989) This method, which relies upon conversion of all of the phosphonate to phosphate, provides excellent results in about twenty to thirty minutes time for phosphonate concentrations down to about 0.02 mg/1 in fresh water, but in brine, as pointed out above, the method is limited to reliable measurements of phosphonates greater than about 5 mg/l.
The desirability of oxidation of complex or polyphosphates to orthophosphates for quantitative analysis has been recognized and various improved procedures suggested. U.S. Pat. No. 3,574,551 teaches disadvantages of oxidation by boiling in sulfuric acid and suggests treatment with a first aqueous solution of a water soluble ferric salt, an alkali metal halide, and a lower fatty acid having 1 to 6 carbon atoms followed by treatment with a second solution of hydroxybenzoic acid, and a lower fatty acid. U.S. Pat. No. 4,544,639 teaches oxidation of organic phosphonates by an oxidizing agent stronger than nitric acid, such as perchloric acid, potassium permanganganate, hydrogen peroxide, potassium dichromate, ozone, sodium bismuthate and ammonium peroxydisulfate in the presence of a silver ion catalyst. U.S. Pat. No. 4,741,400 teaches oxidation of phosphonates to orthophosphate by UV irradiation in the presence of potassium persulfate.
The recovery of phosphoric acid from phosphate rock including first decomposing the phosphate bearing ore with aqueous hydrochloric acid followed by extraction by water from a water insoluble organic extractant is taught by U.S. Pat. Nos. 3,449,074 and 4,353,877.
The recovery of phosphoric acid from aqueous solutions using methyl iso-butyl ketone is known, as exemplified by U.S. Pat. Nos. 3,342,580; 3,914,382; 4,353,877; and 4,127,640. U.S. Pat. No. 4,377,562 teaches solvent extraction of phosphoric acid from aqueous solutions containing at least 45 weight percent phosphoric acid using methyl iso-butyl ketone/cyclohexanol in 50/50 volume percent mixture in combination with sulfuric acid. U.S. Pat. No. 3,449,074 teaches recovery of phosphoric acid from phosphate rock involving a water immiscible extraction agent which may include cyclohexanol or ethyl acetate in which the phosphoric acid-extraction agent solution is contacted with an aqueous solution of hydrogen peroxide providing removal of titanium and vanadium from the phosphoric acid. U.S. Pat. No. 4,524,054 teaches leaching of phosphate ore with a mixture of water, sulfur dioxide and a carbonyl compound which may include methyl iso-butyl ketone.
The use of ammonium molybdate and a reducing agent to form the orthophosphate/molybdate complex imparting a blue color for absorbance measurements is known. U.S. Pat. No. 3,795,484 teaches reduction of the phosphate/molybdate complex with stannous chloride to heteropolyacid having color absorbance of about 700 nm on a spectophotometer. U.S. Pat. No. 3,796,543 teaches an improvement of reduction of the phosphate molybdate by hydrazine to prevent precipitation. U.S. Pat. Nos. 4,544,639 and 4,741,400 teach addition of molybdate or vanadate ion and reduction with a reducing agent such as ascorbic acid followed by measurement of absorbance at 625-800 nm to determine the quantity of orthophosphate.